[Feature Request] add auth.method = "none" to allow frps plugins to be the sole authentication mechanism

Problem

When an HTTP login plugin is configured, frps calls the plugin first and then always runs the built-in VerifyLogin check afterward (server/service.go). There is no way to configure frps to skip the built-in check and delegate authentication entirely to the plugin.

This makes it impossible to implement the natural use case: a login plugin that validates a bearer token (e.g. a signed JWT) without also maintaining a pre-shared token on frps.

Concrete example

• frpc uses auth.method = oidc + auth.oidc.tokenSource = exec to fetch a signed JWT dynamically and send it as privilege_key
• A login plugin validates the JWT (checks signature, issuer, audience, claims)
• frps rejects the connection anyway because VerifyLogin computes MD5(server_token + timestamp) and it does not match the JWT string

As a workaround, I currently make my plugin rewrite privilege_key in its response to MD5("" + str(timestamp))

Proposed solution

Add auth.method = none to AuthServerConfig. When set, NewAuthVerifier returns AlwaysPassVerifier, making VerifyLogin a no-op. Authentication is then fully delegated to any registered login plugins.

The change is small and non-breaking:

• pkg/config/v1/common.go: add AuthMethodNone AuthMethod = none
• pkg/config/v1/validation/validation.go: add none to SupportedAuthMethods
• pkg/auth/auth.go: add case v1.AuthMethodNone: authVerifier = AlwaysPassVerifier
• pkg/config/v1/server.go: keep default as token so existing configs are unaffected

frps config usage:

  [auth]
  method = "none"

  [[httpPlugins]]
  name = "my-auth-plugin"
  addr = "http://127.0.0.1:8080"
  ops = ["Login"]

Why this is safe

A login plugin already has full control over whether a connection is accepted or rejected. VerifyLogin running afterward adds nothing when a login plugin is registered: the plugin is the gate. auth.method = none simply makes that explicit and removes the redundant check.

Affected area

• Docs
• User Experience
• Server Plugin